The transit method detects exoplanets as they pass in front of their parent star, reducing the amount of light reaching telescopes on, and orbiting, Earth.

Although the transit method isn't the best method for detecting exoplanets, it provides a reliable estimate of its size and mass.

Light curves

PhD student Stefan Czesla of the Hamburg Observatory in Germany, and colleagues, examined the giant exoplanet Corot-2b, using data from the French COROT satellite.

Discovered in 2007, Corot-2b is three and a half times the mass of Jupiter and orbits its star in just 1.74 days.

After closely examining the light curves recorded by COROT, which involved splitting them into their red, green and blue components, the researchers determined that the exoplanet is 3% bigger than previously thought.

Czelsa and colleagues believe this discrepancy may be true for other exoplanets around active stars.

"For planets found around active stars the determination of their exact physical parameters is considerably complicated by stellar activity," says Czesla.

Bright and dark spots on the star can modify the transit light curves, something that isn't accounted for in models currently used to calculate an exoplanet's size, he says.

Limitations

Professor Chris Tinney from the University of New South Wales in Sydney, who is undertaking a search for exoplanets at the Anglo-Australian Observatory, says the finding was "interesting [but] won't make huge changes in our understanding of the field".

He says astronomers will, "probably take the effects of stellar activity into consideration, but I don't think its going to change the answer very much."

Dr John Greenhill of the University of Tasmania believes the research also highlights the limitation of the transit method in detecting exoplanets, particularly those smaller than Jupiter.

"The two techniques that have netted the most planets, the radial velocity technique and the transit method, are limited by the noisiness of stars," says Greenhill.

"In principle it looks like we won't be able to detect planets [the size of] Neptune and Uranus, and even Saturn using these methods because of that limit."